There are many types of life on Earth.Life evolved into many wonderful shapes and countless strategies for survival, from single-celled organisms to creatures made up of trillions of cells.The cell theory states that all living things are made up of cells.The term unicellular applies to organisms that have just a single cell.Multicellular organisms are those containing more than one cell.Despite the wide variety of organisms, there are only two basic types of cells: prokaryotic and eukaryotic.Eukaryotic cells have membrane-bound internal structures, called organelles (see chp 2.3).As a result, if you analyzed the cells of any organism found on Earth, you would find either prokaryotic or eukaryotic cells, depending on the organism.

Biologists name, group, and classify organisms based on similarities in genetics and morphology. This branch of biological science is known as taxonomy. Taxonomists group organisms into categories that range from very broad to very specific (Figure 1). The broadest category is called domain and the most specific is species (notice the similarities between the words specific and species). Currently, taxonomists recognize three domains: Bacteria, Archaea, and Eukarya. All life forms are classified within these three domains.

Figure 1. This illustration shows the taxonomic groups, in sequence, with examples. This illustration by OpenStax is licensed under CC BY 4.0

Domain Bacteria

A member of the bacterial domain is a prokaryotic, unicellular organism (Figure 2).There are over a billion of them on earth and they are found in almost every habitat.In fact, most bacteria are harmless or beneficial to humans, despite the misconception that they cause disease.In fact, some bacteria can cause disease in people, but this is more of an exception than a rule.

Metabolism is a well-known characteristic of bacteria.Biochemistry inside cells is called metabolism.Autotrophs are organisms that can provide food for themselves without eating another organism.A majority of autotrophic bacteria convert light energy into chemical energy that is utilized by the cell by photosynthesis.Photosynthesis bacteria such as cyanobacteria are well known and ecologically important.Occasionally, they are called blue-green algae, but that is not the correct name, since, as we shall see shortly, algae are organisms that belong to the domain Eukarya.Among aquatic systems, such as lakes, cyanobacteria play an important role in food webs.

The other type of bacteria is known as a heterotroph, which means they need to eat other organisms in order to produce food.Bacteria that cause disease in humans are included in this classification (during an infection, bacteria eat you).Generally, however, heterotrophic bacteria are harmless to humans.On your skin and in your large intestine live hundreds of species of bacteria that will not harm you.In ecosystems, heterotrophic bacteria play a key role, particularly soil-dwelling bacteria which decompose living matter and assist plants in acquiring nutrients.

2. (Figure).According to their shape, many prokaryotes can be categorized as follows: (a) spherical cocci; (b) rod-shaped bacilli; and (c) spiral-shaped spirilla.The following credit is given to Janice Haney Carr, Dr. Richard Facklam, CDC; Dr. David Cox and Matt Russell who modified the work.By OpenStax, this figure is licensed under CC BY 4.0

Domain Archaea

The Archaea domain contains prokaryotic and unicellular organisms, like bacteria.It is hard to tell what they are by looking at them, but they look a lot like bacteria.Modern DNA analysis has just revealed something about archaeans: they are so genetically different that they belong in their own domain.

Many archaean species are found in some of the most inhospitable environments, areas of immense pressure (bottom of the ocean), salinity (such as the Great Salt Lake), or heat (geothermal springs). Organisms that can tolerate and even thrive in such conditions are known as extremophiles. (It should be noted that many bacteria are also extremophiles). Along with genetic evidence, the fact that a large percentage of archaeans are extremophiles suggests that they may be descendants of some of the most ancient lifeforms on Earth; life that originated on a young planet that was inhospitable by today’s standards.

For whatever reason, archaeans are not as abundant in and on the human body as bacteria, and they cause substantially fewer diseases. Research on archaeans continues to shed light on this interesting and somewhat mysterious domain.

Domain Eukarya

This domain is most familiar to use because it includes humans and other animals, along with plants, fungi, and a lesser-known group, the protists. Unlike the other domains, Domain Eukarya contains multicellular organisms, in addition to unicellular species. The domain is characterized by the presence of eukaryotic cells. For this domain, you will be introduced to several of its kingdoms. Kingdom is the taxonomic grouping immediately below domain (see Figure 1).

Kingdom Animalia is comprised of multicellular, heterotrophic organisms. This kingdom includes humans and other primates, insects, fish, reptiles, and many other types of animals. Kingdom Plantae includes multicellular, autotrophic organisms. Except for a few species that are parasites, plants use photosynthesis to meet their energy demands.

Kingdom Fungi includes multicellular and unicellular, heterotrophic fungi. Fungi are commonly mistaken for plants because some species of fungi grow in the ground. Fungi are fundamentally different from plants in that they do not perform photosynthesis and instead feed on the living matter of others. Another misconception is that all fungi are mushrooms. A mushroom is a temporary reproductive structure used by some fungal species, but not all. Some fungi take the form of molds and mildews, which are commonly seen on rotting food. Lastly, yeast are unicellular fungi. Many species of yeast are important to humans, especially baker’s and brewer’s yeast. Through their metabolism, these yeast produce CO2 gas and alcohol. The former makes bread rise and the latter is the source for all alcoholic beverages.

Figure 3. The (a) familiar mushroom is only one type of fungus. The brightly colored fruiting bodies of this (b) coral fungus are displayed. This (c) electron micrograph shows the spore-bearing structures of Aspergillus, a type of toxic fungi found mostly in soil and plants. (credit a: modification of work by Chris Wee; credit b: modification of work by Cory Zanker; credit c: modification of work by Janice Haney Carr, Robert Simmons, CDC; scale-bar data from Matt Russell). This work by OpenStax is licensed under CC BY 4.0.

Protists refer to a highly disparate group that was formerly its own kingdom until recent genetic analysis indicated that it should be split in to many kingdoms (Figure 4). As a group, protists are very diverse and include unicellular, multicellular, heterotrophic, and autotrophic organisms. The term ‘protist’ was used as a catchall for any eukaryote that was neither animal, plant, or fungus. Examples of protists include macroalgae such as kelps and seaweeds, microalgae such as diatoms and dinoflagellates, and important disease-causing microbes such as Plasmodium, the parasite that causes malaria. Sadly, malaria kills hundreds of thousands of people every year.

See figure 4. .OpenStax' work is licensed under CC BY 4.0

With this cursory and fundamental understanding of biological diversity, you are now better equipped to study the role of biodiversity in the biosphere and in human economics, health, and culture. Each life form, even the smallest microbe, is a fascinating and and complex living machine. This complexity means we will likely never fully understand each organism and the myriad ways they interact with each other, with us, and with their environment. Thus, it is wise to value biodiversity and take measures to conserve it.